1. Field of the Invention
The present invention relates to a piezoelectric inverter for driving a load using a piezoelectric transformer, and, more particularly, to a piezoelectric inverter that is preferably used as a lighting circuit for a discharge tube, such as a cold-cathode tube for use in a liquid-crystal backlight.
2. Description of the Related Art
Small cold-cathode tubes are conventionally used as an backlight illumination source for a liquid-crystal display apparatus. To drive a cold-cathode tube, a piezoelectric transformer, rather than a magnetic transformer, is used because of the compact design and low cost thereof.
Japanese Unexamined Patent Publication No. 7-220888 discloses a driver of a backlight cold-cathode tube employing a piezoelectric transformer. According to this disclosure, a chopper circuit is connected between a direct-current power source and an inverter driving the piezoelectric transformer. The piezoelectric transformer is connected to the cold-cathode tube, and a current flowing through the cold-cathode tube is detected by a tube current detector circuit. The luminance of the cold-cathode tube is maintained constant by controlling a duty factor of the chopper circuit to maintain the tube current constant.
Japanese Unexamined Patent Publication No. 9-107684 discloses a piezoelectric transformer drive circuit which controls a tube current to a desired value by making use of frequency-versus-gain characteristics of the piezoelectric transformer. Connected between an input terminal and the piezoelectric transformer are a drive voltage control circuit having no parts for rectifying and smoothing, and a voltage multiplication circuit. The drive voltage control circuit maintains constant a mean input voltage applied to the voltage multiplication circuit. A cold-cathode tube is connected to the piezoelectric transformer. Also provided is a frequency control circuit which detects a current flowing through the cold-cathode tube, and controls the tube current to a desired value taking advantage of the frequency-versus-gain characteristics of the piezoelectric transformer.
When the input voltage to the voltage multiplication circuit increases with no drive voltage control circuit employed in the control method using the frequency-versus-gain characteristics of the piezoelectric transformer, a drive voltage frequency of the piezoelectric transformer shifts to a high frequency side where the voltage multiplication ratio or gain of the piezoelectric transformer is small, thereby cancelling the increase in the input voltage. The conversion efficiency of the piezoelectric transformer drops in a frequency region where the voltage multiplication ratio is small. In this conventional art, the drive voltage control circuit maintains constant the mean voltage to the voltage multiplication circuit, thereby maintaining the drive voltage frequency of the piezoelectric transformer to a frequency at which efficiency is high. It is therefore believed that the conventional art keeps a relatively high efficiency within a wide input voltage range.
In the conventional art disclosed in Japanese Unexamined Patent Publication No. 7-220888, the output of the chopper circuit is a direct current, and the chopper circuit is thought to be a DC-DC converter. To construct the chopper circuit of a DC-DC converter, inductors and capacitors, for rectifying and smoothing, are required. The component count of the circuit increases, and loss attributed thereto is also increased.
The piezoelectric transformer drive circuit, disclosed in Japanese Unexamined Patent Publication No. 9-107684, needs no rectifier circuit, thereby avoiding the loss attributed thereto.
The conventional art, disclosed in Japanese unexamined Patent Publication No. 9-107684, however, needs two types of feedback control: 1) frequency control for maintaining constant the tube current through a frequency control circuit, and 2) pulse width duty factor control through the drive voltage control circuit for maintaining constant the voltage input to the voltage multiplication circuit. The control circuit therefore becomes complicated, increasing costs involved.